When producing liquid hydrocarbons (oil) from a reservoir the use of a gas lift device is common within the industry. FIG. 1 shows a diagram of an oil well with a single gas lift device fitted.
In FIG. 1, an oil well extends down to an oil reservoir 2 and contains a gas tube 4. Oil is able to flow into the tube 4 via perforations 6 close to the base of the tube. An oil-producing tube 8 extends centrally along the gas tube 4 so that an elongated annular space 10 is defined between the two tubes. A packer 12 forms a seal between the two tubes, above the perforations 6. A gas injection point 14 allows gas pumped into the annular via inlet 16 to be injected into the oil-producing tube 8.
Oil in a reservoir is generally at too low a pressure to flow freely to the surface and therefore needs some kind of artificial lift to ensure this. Gas injection is one such method which works by injecting gas at pressure into the oil producing tube 8, which has the effect of making the column of oil lighter and therefore it rises further up the tube. This in turn allows more gas to be injected and the entire column will then get lighter causing a continuous flow of oil. The gas can subsequently be removed from the oil by a separator (not shown).
When producing oil from a reservoir, the oil produced may be cut with other chemicals such as water, sulphides and many others. These impurities can lead to corrosion in the well bore and also to scaling within the walls of the oil producing well. Chemicals are used to protect the walls of the tubing and to reduce or remove scaling.
A known method for performing chemical injection is to have a dedicated mandrel that has a port for injecting chemicals.
These injection valves work by increasing the pressure of the injection fluid to be above that of the reservoir to force open the valve so that it allows the fluid to pass through. To stop the flow, the fluid pressure is reduced to allow the valve to close. Pressure control of the valve in this way means that the downhole valve mechanism is relatively simple and therefore more durable and reliable. However, this approach requires a dedicated supply line to each of the injection points along an oil producing tube.